Nearly every cell in an animal's body, from neural to blood to bone, owes its existence to a stem cell. A stem cell is commonly defined as a cell that (i) is capable of renewing itself; and (ii) can give rise to more than one type of cell (that is, a differentiated cell) through asymmetric cell division. F. M. Watt and B. L. M. Hogan, “Out of Eden: Stem Cells and Their Niches,” Science, 284, 1427-1430 (2000). Stem cells give rise to a type of stem cell called progenitor cells; progenitor cells, in turn, proliferate into the differentiated cells that populate the body.
The prior art describes the development, from stem cell to differentiated cells, of various tissues throughout the body. U.S. Pat. No. 5,811,301, for example, the disclosure of which is hereby incorporated by reference, describes the process of hematopoiesis, the development of the various cells that comprise blood. The process begins with what may be a pluripotent stem cell, a cell that can give rise to every cell of an organism (there is only one cell that exhibits greater developmental plasticity than a pluripotent stem cell; this is a fertilized ovum, a single, totipotent stem cell that can give rise to an entire organism when implanted into the uterus). The pluripotent stem cell gives rise to a myeloid stem cell. Certain maturation-promoting polypeptides cause the myeloid stem cell to differentiate into precursor cells, which in turn differentiate into various progenitor cells. It is the progenitor cells that proliferate into the various lymphocytes, neutrophils, macrophages, and other cells that comprise blood tissue of the body.
This description of hematopoiesis is vastly incomplete, of course: biology has yet to determine a complete lineage for all the cells of the blood (e.g., it is has yet to identify all the precursor cells between the myeloid stem cell and the progenitor cells to which it gives rise), and it has yet to determine precisely how or why the myeloid cell differentiates into progenitor cells. Even so, hematopoiesis is particularly well studied; even less is known of the development of other organ systems. With respect to the brain and its development, for example, U.S. Pat. No. 6,040,180, the disclosure of which is hereby incorporated by reference, describes the “current lack of understanding of histogenesis during brain development.” U.S. Pat. No. 5,849,553, the disclosure of which is hereby also incorporated by reference, describes the “uncertainty in the art concerning the development potential of neural crest cells.”
The identification and isolation of stem cells has daunted researchers for decades. To date, no one has identified an individual neural stem cell or hematopoietic stem cell. F. H. Gage, “Mammalian Neural Stem Cells,” Science, 287, 1433-1488 (2000). There are two principal difficulties. First, stem cells are rare. In bone marrow, for example, where hematopoiesis occurs, there is only one stem cell for every several billion bone marrow cells. G. Vogel, “Can Old Cells Learn New Tricks?” Science, 287, 1418-1419 (2000). Second, and more importantly, researchers have been unable to identify molecular markers which are unique to stem cells; to the typical immunoassay, most stem cells look like any other cell. Id. Compounding this problem is that primitive stem cells may be in a quiescent state. As a result, they may express few molecular markers. F. H. Gage, supra.
A method to effectively isolate stem cells and culture them in clinically significant quantities would be of immense importance. Researchers are already transplanting immature neurons, presumed to contain neural stem cells, from human fetuses to adult patients with neurodegenerative disease. The procedure has reduced symptoms by up to 50% in patients with Parkinson's disease in one study. M. Barinaga, “Fetal Neuron Grafts Pave the Way for Stem Cell Therapies,” Science, 287, 1421-1422 (2000). Many of the shortcomings of this procedure, including the ethical and practical difficulties of using material derived from fetuses and the inherent complications of harvesting material from adult brain tissue, could be addressed by using cultures of isolated stem cells, or stem cells obtained from adult individuals. D. W. Pincus et al., Ann. Neurol. 43:576-585 (1998); C. B. Johansson et al., Exp. Cell. Res. 253:733-736 (1999); and S. F. Pagano et al., Stem Cells 18:295-300 (2000). However, the efficient and large-scale generation of neural progenitor cells for use in the treatment of neurological disorders has been a challenge.
Recent evidence has suggested that progenitor cells outside the central nervous system and bone marrow cells in particular may have the ability to generate either neurons or glia in vivo. J. G. Toma et al., Nat. Cell Biol. 3:778-783 (2001); E. Mezey et al., Science 290:1779-1782 (2000); T. R. Brazleton et al., Science 290:1775-1779 (2000); and M. A. Eglitis et al., Proc Natl. Acad. Sci. 94:4080-4085 (1997). Bone marrow stromal cells have also been shown to be capable of differentiating into neurons and glia in vitro after a complicated and time-consuming culture process spanning several weeks. The generation of neural progenitor cells from whole bone marrow has, however, not been reported.